It makes a huge diffrence especially going forward in the next couple of years when games are going to be optimized to use the extra memory coming out in higher end video cards. For low resolution gaming right now you may not notice a huge diffrence, but next year when you are getting the extra FPS you will be very glad to have the extra memory.
I see that there is almost negligible FPS between 1gb and 2gb(according to Charts from reviews) even on high resolution. Though, I would still get the 2gb because of Eyefinity, as well as what the future may bring.
People have been telling us that you can't unlock anything but the reference models, but I am able to unlock the MSI R6950 Twin Frozr II. The card would not work with a reference bios either.
However, the shader unlock just requires you to alter the existing bios where it determines how many shaders are unlocked. I see no reason it wouldn't work, and I've also heard there are people that have been successful doing it with the 1gb versions.
You cannot flash the 1gb models to an HD6970 but you can unlock the shaders. The same basically applies to the non-reference 2gb models as well and even for the reference model it's the safer way to go about things. Unlock the shaders with a BIOS flash then overclock on your own with something like Afterburner.